pub const FnInfo = struct {
    param_body: []const Inst.Index,
    param_body_inst: Inst.Index,
    ret_ty_body: []const Inst.Index,
    body: []const Inst.Index,
    ret_ty_ref: Zir.Inst.Ref,
    ret_ty_is_generic: bool,
    total_params_len: u32,
    inferred_error_set: bool,
}